Because there is a wide variety of applications for mechanical packing and seals, including packing for pumps, valves, hydraulic, and pneumatic equipment, a whole industry has grown up in their design and construction. In the areas with which the present invention is concerned, the packing is generally sold in relatively long coils of braided packing material of square or rectangular cross-section from which many suitable lengths may be cut. Conventionally, several lengths are cut from the coil of material for a given installation, each length being formed into a ring about a shaft with the cut ends abutting each other. Often, as many as a half dozen such rings or more are disposed about the shaft with their radial sides in abutting relationship.
A so-called packing or stuffing box formed integrally with, and generally extending outwardly from, the housing surrounds the shaft. The interior of the stuffing box is of a diameter sufficiently greater than that of the shaft to accommodate the packing rings. An annular gland is fitted about the shaft and bolted to the exterior of the stuffing box in such a fashion that an end of the gland compresses the packing rings in the stuffing box. Generally, the gland has a flange through which bolts pass which are threaded into the stuffing box. Tightening of the bolts pulls the gland toward the housing and thereby compresses the packing rings within the stuffing box. Under such compression, the materials tend to expand radially to some extent and substantially fill the stuffing box to prevent or minimize the escape of the contents of the housing at the intersection of the shaft and the housing.
In the original conventional manufacture of the packing material, it is braided in the form of a relatively long, straight length with square or rectangular cross-section. Because it is then cut to desired short lengths, each of which is formed into a ring about a cylindrical shaft, the outside circumference of each packing ring is longer than the inside circumference. Before compression from the gland is applied, the cross-section of each ring tends to form itself into a trapezoidal shape, the narrow side of the trapezoid being the stretched side adjacent the inner surface of the stuffing box. Conversely, the wide side of the trapezoid abuts the shaft. Because of the trapezoidal cross-section which is assumed by each packing ring, this phenomenon is known as "keystoning."
Several problems arise from this keystoning effect. First and foremost, when a plurality of abutting rings are used in a typical application and the packing is compressed by the gland, the force exerted by the gland is concentrated on the packing ring corners adjacent the shaft. This creates a higher normal force between the corners of the packing rings and the shaft adjacent thereto than is created along the ring at locations remote from the corners.
Moreover, wear of both packing and shaft becomes more rapid at these corners adjacent the shaft and foreshortens the usable life of both the packing ring set and shaft and reduces the time before leakage becomes intolerable, necessitating adjustment of gland compression and subsequent replacement of the packing and shaft in a shorter than desirable time.
More specifically, most mechanical compression packing is formed on plait or so-called "lattice" braiding machines, e.g., interlocking braiding machines, by braiding yarns from moving yarn carriers about axial warp yarns in such a fashion that the warp yarns provide fill which is symmetrical about the central axis of the finished braid. Said otherwise, the warp yarns, when viewed cross-sectionally, are conventionally the same in number and by position when any 90-degree quadrant of a cross-section of the packing is viewed in mirror image compared to the adjacent two quadrants. The cross-section of the conventionally braided packing has a square or rectangular shape when a length of the material is placed on a flat surface, and the opposite sides of the square or rectangle are generally parallel and equal in width.
However, when measured lengths of material are wrapped about a cylindrical body, such as a pump sleeve or valve shaft, to form rings with outer sides abutting the inner surface of a stuffing box, the outer sides are placed under some circumferential tension. As a result, each ring's outer side, i.e., the side adjacent the interior of the stuffing box, tends to contract in the direction parallel to the axis of the ring before the rings are compressed by the gland. Also, the inner side, i.e., the side adjacent the surface of the shaft, tends to expand in length in the direction parallel to the ring axis. When the rings are compressed, more of the load is directed axially to the inner portion of the packing rings adjacent the shaft. The force is concentrated at the abutting inner corners of the rings, and this results in unwanted wear.
Various alternatives have been proposed to combat the unwanted effects of keystoning. One alternative involves the use of packing rings which are interspersed with compensating rings machined or die-formed into wedge shapes having the wider axial dimension at the outer diameter. The packing rings are then installed in proper sequence with the "wedge spreaders" to compensate for keystoning. This expedient is useful and has provided some relief from sealing problems, but it is expensive and requires special forms of packing rings and wedges which must be carefully assembled and installed in the proper order.
Moreover, in the past there has been a problem when trying to pack rings of small cross-section such as those in the one-quarter inch to the five-sixteenths inch range about relatively small diameter shafts of one-half to one and three-eighths inches. The problem is that when the packing is wrapped around the shaft, the warps which are at the interior corners tend to pop out and result in shaft scarring. This can occur either before or after gland pressure is applied. Moreover, warp popping can occur in manufacture when these small cross-section braids are drawn off the braiding machine around a capstan.
As will be appreciated, the popping problem relates to certain particular materials used in the construction of the braided packing. If a material of high strength and relatively low yield is used in the construction of the packing, then the yarns used as the corner fill on the inside of the packing, the side which will be placed adjacent the shaft, will tend to "pop" out through the braided outer layer, as there is a resistance to their compressing axially within the braid. Thus, the inner axial corner yarns tend to deform to the side instead of remaining constrained within the braid. This "popping" occurs because a high intensity point load is placed upon the braid when the braid passes over a radiused surface and results in what appears to be a loop of axial yarn outside the braid. Examples of packing materials which have this tendency are Kevlar IT aramid packing yarn which is a plied aramid monofilament yarn impregnated with Teflon, i.e., polytetrafluorlethylene, and treated with silicone, i.e., dimethylsiloxane, manufactured by E. I. DuPont Company, and graphite filament yarn cordage which may or may not contain various sizings, finishes or treatments manufactured from plied and twisted rayon monofilament precursor yarns or spun, twisted and plied "pitch" based graphite yarns as manufactured by the Polycarbon Company.
When packing containing these yarns "pops" during manufacture, or when the "pops" are created as result of placing the packing in the stuffing box, if the "pop" becomes located between the packing and the shaft or shaft sleeve, then a local high intensity point load is placed upon the shaft surface as a result of this "popped" warp. This results in much higher than normal wear on the shaft or shaft sleeve and has resulted in wear so high that one can see evidence in worn sleeves in which the cuts resemble those made by a machine tool operating on the surface of a rotating shaft.
While the popping problem is particularly severe for packings used about small diameter shafts, popping is also a problem in packing made with these materials for use on larger shafts. It is therefore a primary objective of the present invention to prevent such "warp pop-out" so as to protect shafts from scarring.
It is also an object of this invention to provide a tapered density profile for the packing ring which better eliminates the aforementioned "keystoning" effect when the packing ring is wrapped around a shaft and is compressed by providing a more regular initial trapezoidal cross-section.
Another object of the present invention is to avoid the use of packing rings and wedges or preshaped cross-sections which must be assembled in a particular sequence.
Still another object of the invention is to avoid the need of packing rings which are die-formed so that they are preshaped to fit the stuffing box.
A further object of the invention is the production of mechanical compression packing which is not subject to either keystoning or warp popping.